Canadian Forest Service Publications

Decomposition, delta 13C, and the “lignin paradox” 2006. Preston, C.M.; Trofymow, J.A.; Flanagan, L.B. Canadian Journal of Soil Science 86: 235-245.

Year: 2006

Issued by: Pacific Forestry Centre

Catalog ID: 26211

Language: English

Availability: Order paper copy (free), PDF (download)

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Abstract

The natural abundance of 13C (delta 13C) generally increases with decomposition of organic matter. This is contrary to the expected decrease, as lignin is hypothesized to accumulate relative to isotopically heavier cellulose. Our objective was to test the hypothesis that 13C depletion should be observed for gymnosperm logs that typically develop advanced brown-rot decay with high lignin content. With increasing lignin concentration [previously determined by nuclear magnetic resonance (NMR)], delta 13C tended to become more negative for samples of Pseudotsuga menziesii, Tsuga heterophylla, Thuja plicata, and unidentified species from Coastal Forest Chronosequence sites of southern Vancouver Island. For a larger sample set without NMR analysis, delta 13C was significantly more depleted for the highest decay classes, and total C was negatively correlated with delta 13C, consistent with the higher total C of lignin than of cellulose. Relationships of total C and delta 13C with density were much weaker. We discuss causes for the variability of delta 13C in coarse woody debris from these sites, and how the apparent paradox in the predicted change of delta 13C with decomposition is largely due to the confusion of lignin, the biopolymer produced by higher plants, with the acid-unhydrolyzable residue (AUR) of the proximate analysis procedure commonly used to assess litter quality in decomposition studies.